Accurate absorption intensities of ozone in a wide IR spectral range from experimental measurements and ab initio calculations

. Recent results of a long-term effort of Tomsk-Reims research teams on accurate intensity data obtained from high-resolution absorption spectra of ozone suitable for the atmospheric remote sensing in a wide IR range are summarized. This includes vibronic bands corresponding to the transitions towards triplet electronic states near 1 µm, and vibration-rotation bands covering the far-infrared and near-infrared ranges.


Introduction
Ozone (O3) plays an important role for protecting life on Earth and in the climate formation.On the other hand, it acts as a harmful pollutant in the troposphere that must be controlled.Accurate spectroscopic data are mandatory for the global remote sensing of ozone using satellite and ground based observations.For a reliable determination of the real-time ozone variation in the atmosphere it is crucial to have precise and reliable laboratory data in a wide spectral range to make it possible observations with independent instruments.We summarize recent results of our groups aimed at providing consistent data both for far-and mid-infrared vibration-rotation bands and for vibronic bands near 1 micron.

Absorption cross-sections in the near IR range of Wulf bands measured using the cw-CRDS laser spectrometer
The absorption in the Wulf bands near 1 micron are due to the transitions from the electronic ground state (GS) towards rovibrational levels of the 3 A2, 3 B2 and 3 B1 triplet states just above the GS dissociation energy of 1.06 eV.These bands have been studied in [1,2] (and refs therein), however the reported in [2] line intensities were contradictory [3].The absolute values of the absorption cross-sections of the strongest Wulf bands have not been published and are lacking in the spectroscopic databases (HITRAN, GEISA or S&MPO) that are widely used for the atmospheric remote sensing.The aim of our works was to fill this gap.New spectra were measured using recently constructed continuous-wave cavity-ring-down (cw-CRDS) spectrometer [4].As it was demonstrated in [5], this method provided very sensitive measurements for absorption coefficient of ~10 -10 cm -1 that was higher than that in the commercial Fourier spectrometers.Another advantage is the possibility of the direct recording of the absolute value of the absorption coefficient not affected by an instrumental function.The synthesis of O3 was carried out by an impact of a silent discharge on oxygen at a liquid nitrogen temperature of 77 K. Special care was given to stabilize ozone partial pressure to 1 -2 % during the measurements.The external-cavity diode lasers provided the single-mode generation with the radiation bandwidth less than 1 MHz.The spectrum was recorded with the step of about 0.007 cm -1 and the integration time of 0.1 s per one spectral point.cw-CRDS spectra recorded for the first time in the range above 10000 cm -1 with several partial pressures at medium and high resolutions (spectral steps of 0.1 cm -1 and 0.003 cm -1 , respectively) showed a sequence of the narrow lines and broadened features due to the short lifetimes of the upper rovibronic states.A sample of the recorded spectrum is given in Fig 1 .This spectral interval which falls in the water vapor transparency window can be used for the atmospheric remote sensing.A further step of the project is to determine the predissociative line broadening that will give the information on the ozone life times in the excited triplet electronic states.

Sub-percent accuracy of the experimental and ab initio line intensities in the strongly absorbing intervals of the rotational, fundamental and overtone vibrational bands
Ozone an unstable molecule being in a dynamic exchange with the oxygen atoms and reacting with other atmospheric species.This makes difficult a precise quantification of the ozone absorption both in the laboratory cell and in the atmosphere.For an accurate control of the absorption line intensities in vibrationrotation bands, a combined approach was applied in this work.On the experimental side, they were deduced from the Fourier Transform Spectra (FTS) [6] with the intensity calibrations versus UV.On the other hand, they were computed using the high-level ab initio methods [7].Fig. 2. Example of the agreement of the theoretical line intensities (calculated using the ab initio dipole moment surface a# of ref. [7]) with the empirical S&MPO line list based on the FTS measurements in Reims [6] for the strongest 3 band.
A comparison of these independent data has permitted obtaining a sub-percent accuracy for the strongest fundamental and overtone bands (as illustrated in Fig. 2) that was confirmed by comparisons with other accurate experiments [8][9].

Consistency of the band intensity data in various spectral windows from MW to far-IR and mid-IR ranges
Another issue that remained unsolved for many decades is a consistency of the ozone atmospheric retrievals using various spectral intervals.When the laboratory databases were used in the past, the discrepancy could amount up to 15% depending on a selected IR interval [10].The intensity calibration of about thirty bands using our ab initio calculations [11] has permitted to make the spectral line list much more homogeneous.Moreover, the independent experimental validation (by G.Toon [10]) of these results (included in the last version of the S&MPO databank, https://smpo.tsu.ru) has shown that the consistency of the ozone retrieval from 630 to 4900 cm -1 has been significantly improved among 37 spectral windows.This validation has indicated [10] that the window-to-window fluctuations of the retrievals have become lower by factor 1.9 for the balloon spectra, by factor of 2.7 with respect to the previous databases obtained from laboratory spectra and by factor 2.1 for atmospheric ground-based observations.This permitted to reduce the window-towindow intensity discrepancies (computed from the variance mean ratio scaling factors [10]) for these types of experiments to 1.06%, 1.33% and 2.58% correspondingly.